Leadership/Implementation Project Project Summary This application proposes the Washington University in St. Louis School of Medicine (WUSM) Model Organism Screening Core (wuMOSC) as a key asset for Phase II of the NIH Undiagnosed Diseases Network (UDN). The wuMOSC will evaluate the pathogenicity of 200 genetic variants per year identified by UDN Clinical Sites in otherwise undiagnosed participants by leveraging the optimal combination of the following four model organism and cell-based Resource Cores: 1) C. elegans, 2) Drosophila, 3) zebrafish, and 4) human pluripotent stem cells (hPSCs). The combination of these model systems allows for extremely rapid and cost-effective variant evaluation in C. elegans, and evaluation in hPSCs of genes and variants that are not conserved in the animal model organisms, while maintaining the established advantages of the Drosophila and zebrafish models. An experienced Leadership Team has been assembled that harnesses the collaborative research environment at WUSM, including the expertise of the McDonnell Genome Institute, and the superb WUSM clinical partners that constitute the proposed UDN Phase II WUSM Sequencing Center and Clinical Site applications, respectively. Using proven, cutting-edge, and novel bioinformatic approaches, combined with thoughtful consideration of the advantages and limitations of each model organism, a careful assessment plan has been defined for determining the putative pathogenicity of nominated variants and prioritizing them for further evaluation by the Resource Cores. The leaders of the four Research Cores have significant expertise in the relevant model system, and are currently employing the proposed advanced genetic approaches (CRISPR/Cas9 knock-in, tissue-specific RNAi and overexpression, and testing variant mRNA for null mutant rescue) and extensive phenotypic analysis to assess human gene variants for effect on gene product function. The UDN Clinical Sites, Steering Committee, and Coordinating Committee will receive frequent communications regarding plans and results of the wuMOSC, through the activity of the Administrative Core, which will also disseminate acquired model organism expertise to the wider NIH and other research communities. Therefore, the wuMOSC will have an exceptionally high impact on the diagnostic efforts of the UDN by bioinformatically and experimentally screening potential disease-causing variants in the context of disease-specific phenotypes.